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The day after Serena Williams was charged with three infractions, chair umpire Alison Hughes gave Novak Djokovic and Juan Martin del Potro leeway with the 25-second serve shot clock and did not hand Djokovic a code violation for smashing his racket.

The International Tennis Federation defended Carlos Ramos, the chair umpire who gave Serena Williams three code violations during the US Open final, saying his "decisions were in accordance with the relevant rules."

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The duo, though, will not be joined in the second-round by third seed Yulia Putintseva, who was forced to retire hurt through injury after losing the first set of her match-up with Anna Karolina Schmiedlova.

Earlier in the day, home favourite Hibino came through 6-4 6-4 against Bouchard in another surprise result, continuing the former world No. 5's poor form in 2018.

Bouchard, who lost to Marketa Vondrousova in the second-round of the US Open last month, also fell at the first hurdle in the Oracle Challenger Series at the start of September against qualifier Kristie Ahn.